It is state of the art to anneal cold rolled aluminium strips at 250-500° C. The purpose is to restore good formability.
The mechanisms are removal of dislocation pile-ups (partial annealing) and recrystallization (annealing).
The recrystallization process is among others depending on time and temperature. For example at 500° C. recrystallization takes a few seconds, at 380° C. a few minutes and at 280° C. a few hours. Other factors are alloy composition and the amount of cold work prior to the annealing.
The partial annealing take place at 200-300° C. for prolonged times up to 15 hours.
For aluminium strip coils a car bottom box furnace is normally used. The furnace is either heated by electrical elements or by fuel heated elements. To get good convection and temperature homogeneity in the furnace powerful fans are used to circulate the furnace atmosphere. The car bottom box furnace represents a significant investment.
The Direct flame impingement (DFI) technique, where multiple oxyfuel burner flames directly hits and heats a moving steel strip is a technology previously developed and patented. DFI burners are normally fed with fuel and an oxidant having a high oxygen content. It is preferred to use an oxidant having at least 80% by weight oxygen. Using DFI burners provides a high heat transfer from the flame to the steel strip and thus a very high heating rate.
However, DFI burners when fired with an oxidant with a high oxygen content, give a very high output power and a high flame temperature, such as 2500° C.
In spite of this fact it has surprisingly been found out that it is possible to heat an aluminium strip very fast to a desired temperature without suffering from surface damages such as local melting on the surface of the strip. Aluminium has a melting point of approximately 660° C.
There is a problem with annealing according to prior art. Prior art coil annealing is a slow process. It is characterized by inefficient heating and low thermal conductivity between the layers of aluminium strip within the coil. This leads to long process times, low productivity and high energy consumptions.
A second problem is the risk of explosions from evaporated lubricants from the surface of the coiled material igniting with air inside the furnace.
A third problem is discolorations on the strip surface owing to reactions between the rolling lubricant, the metal and the atmosphere.
A forth problem is that a long process time can cause a growth of the oxide layer on the strip surface leading to reduced soldering properties and other negative effects.
A fifth problem is that temperature gradients arise within the coil during the heat treatment. In partial annealing of coils there is a risk that the outer layers of the coil are heat treated at a different time temperature profile than the inner layers and this could lead to variations in mechanical properties.
The present invention solves all of the above mentioned problems.